1. Field of the Invention
The present invention relates to a paper feeding method and a printing apparatus which are used in a computer system and, more particularly, to a paper feeding method of a print paper and a printing apparatus for continuously feeding out sheets by selecting one of a plurality of paper feed cassettes upon printing.
2. Description of the Related Art
For instance, in a laser printer which uses cut sheets and is used as a printing apparatus of a computer system, a plurality of paper feed cassettes are provided in accordance with the sizes of the cut sheets. One of the paper cassettes is selected in accordance with the designation of the paper size from a host computer, the cut sheets of the designated size are fed to a printing section, and image data is printed onto the paper every sheet.
In the laser printer, when the cut sheet arrives at a predetermined position in front of a copy transfer position, a drum writing operation to form a latent image onto a photo sensitive drum is started. For instance, in the case where the paper feed cassettes are enclosed to three stages of the upper stage, middle stage, and lower stage, a time which is required until the cut sheet is fed from each stage to the starting position of the drum writing operation differs.
In case of selecting the lower stage paper feed cassette which needs a long paper feed distance, therefore, a conveying time which is required until the paper reaches the starting position of the drum writing operation is long. A processing time to print becomes slow by only a time corresponding to such a long conveying time, so that it is demanded to improve such a drawback.
FIG. 1 (PRIOR ART) shows a schematic construction of a conventional printing apparatus and relates to a laser printer 10 as an example.
In FIG. 1, an upper stage cassette 28-1, a middle stage cassette 28-2, and a lower stage cassette 28-3 are provided as paper feed cassettes. Papers are fed to a printing position of a printer having a photo sensitive drum 32 by a conveying path shown by a broken line. In the printing operation, a laser beam 36 is emitted from a semiconductor laser 34 of an optical system 20, and a latent image of a print pattern is formed onto the charged photo sensitive drum 32 by a drum unit 38 by the irradiation of the laser beam. A toner is subsequently supplied by a developer 42 and the latent image on the drum is developed. After that, the developed image is transferred onto the cut sheet by a copy transfer section 46. Finally, the transferred image on the paper is fixed by a fixing unit 50 and the paper is fed onto an output tray 52.
A paper sensor 200 is provided at a predetermined position in front of the printing position on the conveying path of the cut sheet. When the paper sensor 200 detects the paper, the writing operation to form a latent image onto the photo sensitive drum 32 is started at a predetermined timing.
In such a conventional printing apparatus using cut sheets as mentioned above, however, the forming process of the latent image onto the photo sensitive drum 32 is started at the timing based on a detection signal of the paper sensor 200 provided on the conveying path after the cut sheet fed from either one of a plurality of paper cassettes 28-1 to 28-3 merged, so that there is a problem such that the processing speed becomes slow in dependence on differences of the paper cassettes 28-1 to 28-3.
FIGS. 2A to 2D show the operation to switch from the upper stage cassette 28-1 to the lower stage cassette 28-3.
In a paper feed state in which the upper stage cassette 28-1 has been selected, when the paper is fed from the upper stage cassette 28-1 on the basis of a pickup instruction signal which is generated at time t.sub.1 in FIG. 2A, a sensor output of the paper sensor 28-1 is obtained at time t.sub.2 in FIG. 2B after the elapse of time T.sub.10. Synchronously with the sensor output, the writing operation onto the photo sensitive drum 32 is started as shown by a write timing in FIG. 2D.
It is now assumed that the writing operation onto the drum is executed at time t.sub.4 by a paper feeding operation based on a pickup instruction signal generated at time t.sub.3 and, after that, the feed cassette to feed a paper is switched to the lower stage cassette 28-3. Since the paper feeding position becomes long due to the switching of the paper cassette, a time which is required until the sensor output is obtained at time t.sub.6 in FIG. 2B after the paper was fed from the lower stage cassette 28-3 on the basis of a pickup instruction signal at time t.sub.5 in FIG. 2C becomes long to be T.sub.20, so that the write start timing to the drum in FIG. 2D is also delayed.
That is, although the writing operation onto the drum is executed every period T.sub.100 which is determined by times t.sub.2 to t.sub.4 in case of feeding the papers from the upper stage cassette 28-1, when the feed cassette is switched to the lower stage cassette 28-3, the papers are fed every period T.sub.200 of times t.sub.4 to t.sub.6 and a writing period to the drum becomes long by only a time corresponding to a long distance of the paper feed cassette which is used, so that a processing speed decreases.